SILGen: Don't allow referencing the 'var' box before it's fully initialized

[slavapestov]

In the following code, SILGen emits a mark_uninitialized instruction
on the projected box value, diagnosing the reference to 'x' before
it has been fully initialized:

var x: Int
x = { _ = x; 123 }()

However, when the binding has an initial value, we don't emit the
mark_uninitialized instruction, and the closure ends up capturing the
uninitialized box:

var x: Int = { _ = x; 123 }() // undefined behavior

Unfortunately, we can't unconditionally emit a mark_uninitialized here,
because DI does not recognize all the code patterns that SILGen can emit
when initializing an address-only value in-place.

Instead, what we want to do is only enter the box into SILGenFunction::VarLocs
after the initial value expression has been emitted. This ensures that the
forward-referenced-capture diagnostics catch this.

Note that this only addresses the case of a local binding; we still
generate invalid code here if you forward-reference a global 'var' in
this manner. Fixing that requires some additional changes.

Fixes https://bugs.swift.org/browse/SR-14747, rdar://problem/79051431, rdar://problem/77933460.

[slavapestov]

@jckarter Does this look reasonable?

[slavapestov]

@swift-ci Please smoke test